BBY/ABY is used to position items along the Star Wars Timeline. BBY refers to Before the Battle of Yavin. ABY refers to After the Battle of Yavin. The Battle of Yavin in Episode IV: A New Hope occured in 0 BBY/ABY.
Some of the activities included also came from the Star Wars comic strip . For example, the activity "Danger in the Desert" (where the reader has to guide Luke back to his house while at the same time avoiding Sandtroopers and Tusken Raiders) was originally included with Princess Leia, Imperial Servant.